Abstract
Silica-supported chloro alkyl aluminum co-catalysts (DEAC@support) were prepared via Surface Organometallic Chemistry by contacting diethylaluminum chloride (DEAC) and high specific surface silica materials, i.e. SBA-15, MCM-41, and Aerosil SiO2. Such systems efficiently activate NiCl2(PBu3)(2) for catalytic ethene dimerization, with turnover frequency (TOF) reaching up to 498,000 moI(c2H4)/(mol(ni) h) for DEAC@MCM-41. A detailed analysis of the DEAC@SBA-15 co-catalyst structure by solid-state aluminum-27 NMR at high-field (17.6 T and 20.0 T) and ultrafast spinning rates allows to detect six sites, characterized by a distribution of quadrupolar interaction principal values C-Q and isotropic chemical shifts.delta(iso). Identification of the corresponding Al-grafted structures was possible by comparison of the experimental NMR signatures with these calculated by DFT on a wide range of models for the aluminum species (mono- versus di-nuclear, mono- versus bis-gkafted with bridging Cl or ethyl). Most of the sites were identified as dinuclear species with retention of the structure of DEAC, namely with the presence of mu(2)-Cl-ligands between two aluminum, and this probably explains the high catalytic performance of this silica-supported co-catalysts. (C) 2014 Elsevier Inc. All rights reserved.